Generally, nonvolatile memories such as magnetic memory and phase change memory (PCM) have a data processing speed of a random access memory (RAM), and a data preserving characteristic even when a power source is off.
FIGS. 1a and 1b are diagrams illustrating a conventional phase change resistor (PCR). A PCR 4 includes a phase change material (PCM) 2 inserted between an upper electrode 1 and a lower electrode 3. When a voltage and a current are applied, a high temperature is induced to the PCM 2 so that an electric conductive state is changed depending on resistance change.
The PCM 2 includes AgInSbTe. As a material of the PCM 2, Chalcogenide having S, Se and Te of chalcogen as a main ingredient, can be used. Specifically, germanium antimony tellurium alloy (Ge2Sb2Te5) consisting of Ge—Sb—Te is used.
FIGS. 2a and 2b are diagrams illustrating a principle of the conventional PCR. As shown in FIG. 2a, when a low current of less than a critical value flows in the PCR 4, the PCM 2 has a proper temperature to be crystallized. As a result, the PCM 2 enters a crystalline phase to become a low resistance material.
As shown in FIG. 2b, when a high current of more than a critical value flows in the PCR 4, the PCM 2 has a temperature over a melting point. As a result, the PCM 2 enters an amorphous phase to become a high resistance material.
The PCR 4 can store nonvolatile data corresponding to the two resistance states. That is, logic states of two data can be stored if when the PCR 4 is in a low resistance state is data “1” and when the PCR 4 is in a high resistance state is data “0”.
FIG. 3 is a diagram illustrating a write operation of the conventional PCR. When a current flows between the upper electrode 1 and the lower electrode 3 of the PCR 4 for a given time, a high heat is generated. As a result, the PCM 2 becomes crystallized and amorphous by a state of temperature applied to the upper electrode 1 and the lower electrode 3.
When a low current flows for a given time, the PCM 2 becomes crystallized by a low heating state so that the PCR 4 which is a low resistor enters a set phase. When a high current flows for a given time, the PCM 2 becomes amorphous by a high heating state so that the PCR 4 which is a high resistor enters a reset phase. As a result, a difference of the two phases is represented as an electric resistance change.
In order to write the set state in a write mode, a low voltage is applied to the PCR 4 for a long time. On the other hand, in order to write the reset state in the write mode, a high voltage is applied to the PCR 4 for a short time.